FIG. 38 illustrates an example of conventional image sensor modules. The image sensor module 900 shown in FIG. 38 serves to read a written content of an object to be read 890 transported in a secondary scanning direction y, as image data. The image sensor module 900 includes a housing 91, a substrate 92, an LED module 93, a light guide 94, a lens unit 95, a sensor IC 96, and a transmission plate 97. The housing 91 has an elongate shape extending in a primary scanning direction orthogonal to the secondary scanning direction y and a thickness direction z. The substrate 92 has an elongate rectangular shape extending in the primary scanning direction, and is fitted in the housing 91. The LED module 93 includes a plurality of LED chips 93a, and is located close to an end portion of the housing 91 in the primary scanning direction.
The light guide 94 serves to emit the light from the LED module 93 toward the object to be read 890, and is formed of a transparent resin material. The light guide 94 has an elongate shape extending in the primary scanning direction, and includes an incidence surface (not shown), a reflection surface 94a, and an output surface 94b. The incidence surface is an end face of the light guide 94 in the primary scanning direction and configured to face the LED chips 93a. The reflection surface 94a serves to reflect the light from the incidence surface. The output surface 94b has an elongate shape extending in the primary scanning direction, and serves to output the light that has proceeded through inside the light guide 94 to the object to be read 890, in a form of a linear light beam extending in the primary scanning direction. The light outputted from the light guide 94 is emitted to the object to be read 890 through the transmission plate 97, and reflected by the object to be read 890. The reflected light is converged into the sensor IC 96 by the lens unit 95. The sensor IC 96 is capable of outputting a signal based on the amount of received light. The written content of the object to be read 890 can be read as an image, by storing the signals from the sensor IC 96 in a non-illustrated memory.
When the object to be read 890 is creased or wrinkled, the object to be read 890 is prone to be lifted from the transmission plate 97. To form a clear image of the written content of the object to be read 890 in the sensor IC 96 under such a condition, it is advantageous to prolong the optical path between the object to be read 890 and the sensor IC 96. However, locating the sensor IC 96 at a lower position in order to prolong the optical path results in an increase in size of the image sensor module 900 in a thickness direction z. Such an increase in size makes it difficult to properly incorporate the image sensor module 900 in a related device, such as a document scanner.
Accordingly, the optical path length may be prolonged by placing a reflecting mirror in the housing 91 thereby bending the optical path. By bending optical path, the optical path can be prolonged by moving the sensor IC 96 in a horizontal direction, instead of moving the sensor IC 96 to a lower position. Therefore, the housing 91 can be formed with a reduced thickness. In this case, however, a cavity has to be provided inside the housing 91 for allowing the light to pass, which leads to a decline in rigidity of the housing 91, even though the housing 91 can be made thinner.